Optical communications networks, at one time, were generally “point to point” type networks including a transmitter and a receiver connected by an optical fiber. Such networks are relatively easy to construct but deploy many fibers to connect multiple users. As the number of subscribers connected to the network increases and the fiber count increases rapidly, deploying and managing many fibers becomes complex and expensive.
A passive optical network (PON) addresses this problem by using a single “trunk” fiber from a transmitting end of the network, such as an optical line terminal (OLT), to a remote branching point, which may be up to 20 km or more. The remote branching point may in turn be optically coupled to destination subscriber units or optical networking units (ONUs). One challenge in developing such a PON is utilizing the capacity in the trunk fiber efficiently in order to transmit the maximum possible amount of information on the trunk fiber. Fiber optic communications networks may increase the amount of information carried on a single optical fiber by multiplexing different optical signals on different wavelengths using wavelength division multiplexing (WDM). In a WDM-PON, for example, the single trunk fiber carries optical signals at multiple channel wavelengths to and from the optical branching point and the branching point provides a simple routing function by directing signals of different wavelengths to and from individual subscribers (ONUs). In this case, each subscriber may be assigned one or more of the channel wavelengths on which to send and/or receive data.
To transmit and receive optical signals over multiple channel wavelengths, the OLT in a WDM-PON may include a multi-channel transmitter optical subassembly (TOSA) and a multi-channel receiver optical subassembly (ROSA). One example of a TOSA includes an array of lasers optically coupled to an arrayed waveguide grating (AWG) to combine multiple optical signals at multiple channel wavelengths. To provide the different channel wavelengths, tunable lasers may be used in the multi-channel TOSA and the wavelengths emitted by the tunable lasers change with changes in temperature. The ONUs in a WDM-PON may include a bi-directional optical subassembly (BOSA) comprising both a TOSA (e.g., a laser diode) for transmission and ROSA (e.g., a photo diode) for reception of these optical signals.
As optical networks increase in coverage and greater numbers of ONUs are deployed in subscriber's homes, e.g., fiber-to-the-home (FTTH), it becomes increasingly important to reduce the cost of the ONU. One challenge to ONU cost reduction is that a separate housing is generally employed for the TOSA (sometimes referred to as a boxed TOSA) in addition to the housing provided for the ONU. This additional housing is typically a hermetically sealed housing (e.g., under a vacuum) which may be one of the more expensive components of the system.
Another challenge is to provide efficient thermal management for the ONUs, and for the laser in particular, since unwanted temperature variations may affect laser tuning and the resulting wavelength of the generated optical signal. Providing adequate temperature control of the laser in a relatively small space and with relatively low power consumption may be difficult. The traditional boxed TOSA design may decrease the thermal management effectiveness of the ONU system by interposing an additional insulating layer between the laser and the thermal management system.